The present invention concerns a device for noise attenuation in a tube intended to transport a gas of the type found in motor vehicles.
More specifically, it concerns a device for noise attenuation of exhaust pipes or of air intake conduits.
FIGS. 1a and 1b depict a well-known noise attenuation device commonly referred to as the xe2x80x9cquarter wave devicexe2x80x9d, consisting, as is discernible in FIG. 1a, of integrating a closed-end branch 2 to the tube to be treated 1.
This type of device enables realization of significant noise attenuation but only over a narrow frequency interval, as is discernible in FIG. 1b, i.e., between 300 and 380 Hz for a tuning frequency, for example, on the order of 340 Hz (a wavelength to be treated of 1000 mm).
FIGS. 2a and 2b depict another device for noise attenuation, commonly referred to as an xe2x80x9cinterference tubexe2x80x9d, consisting, as is discernible in FIG. 2a, of integrating a branch of a second tube 4 with a specific length and diameter to the tube to be treated 3. Such a device advantageously enables treating a noise over a broader frequency interval than that treated by the quarter wave device, however, the attenuation obtained proves not to be very great, as is discernible in FIG. 2b, for the same tuning frequency as above.
An object of the invention is to avoid in particular this type of disadvantage by proposing a device for noise attenuation which is particularly effective over a broad frequency band and which is simple to implement.
To that end, an object of the invention is a device for noise attenuation in a tube intended to transport a gas, characterized in that it consists, on the one hand, of a branch tube of which the two ends open into the tube to be treated, and, on the other, of at least one tube, referred to as quarter wave tube, closed at one of its ends, referred to as the free end, and whose other end is connected to the tube at the level of a connection of the branch tube with said tube, thus forming at least three circulation sections for the gas entering said tube.
When the device according to the invention comprises only one quarter wave tube, it can be connected at the level of one or the other of the connections of the branch tube with said tube.
According to other interesting provisions of the invention:
the length of each section is defined as follows:
L2=xcexxe2x88x920.85 D2 
L3=xc2xdxcexxe2x88x920.85 D3 
L4=L5=xc2xcxcexxe2x88x920.425 D4 
where xcex is the wavelength to be treated and Dn is the tube diameter of the section n.
the diameters of each of the sections are determined such that they are as close as possible to the diameter of the tube to be treated.
the sections and the tube to be treated all have the same diameter.
the diameter of the sections is determined as follows as a function of the diameter of the tube to be treated:
D1=D4=D5 
and D2=D3=xc2xeD1. 
the free ends of each of the tubes referred to as quarter wave tubes are connected to each other.
Another object of the invention concerns a gas conduit for a motor vehicle engine of the air intake conduit type, characterized in that it has at least one device for noise attenuation according to the present invention.